This invention relates to an encoding method and apparatus for bit compressing digital audio signals and, more particularly, to an encoding method and apparatus for changing bit allocation of a plurality of processing blocks in accordance with the energy shape in the processing blocks.
Among the techniques of bit compressing the input digital audio signals, there is such technique as disclosed in EP application Publication Number 0525809 A2 (date of publication of application, Mar. 2, 1993 Bulletin 93/05). According to this encoding technique, division of the block floating units is made along the frequency scale based upon the critical bands and sub-bands produced on further dividing or combining the critical bands. In view of psychoacoustic characteristics of the human hearing mechanism, it is most desirable to use units along the frequency scale which are in register with the critical bands. However, since the critical band widths become broader and narrower towards the higher and lower frequency ranges, respectively, the bands obtained by sub-dividing the critical bands are used for the higher frequency range and the critical bands or the bands combined from critical bands are used for the lower frequency range. For encoding, the values indicating the states showing the block floating states and the scale factors are calculated from one block floating unit to another and bit allocation is performed from one block floating unit to another using these values and the scale factors.
However, if the bands obtained by sub-dividing or combining the critical bands are used as the block floating units, the amounts of the masking effects in the critical bands cannot be calculated correctly, such that sufficient masking effects cannot be realized.